Large fluid swivels with a sealing surface diameter typically between two to eight feet, that carry fluid at a high pressure, (over 1000 pounds per square inch), are used in offshore hydrocarbon floating production systems. In such a system, hydrocarbons such as oil and natural gas as well as other fluids, are transferred between an undersea well or pipeline and a ship, the fluid swivel allowing the ship to weathervane (turn with changing winds, waves and currents). Such fluid swivels often include inner and outer ring-shape swivel parts rotating on one another. An annular chamber is formed between the swivel parts, and a pair of gap passages extend from the chamber to the environment. One or more seal arrangements lie along each gap passage to prevent the leakage of pressured fluid into the environment. Each seal arrangement includes a seal assembly lying in a recess in one of the swivel parts (usually the outer swivel part) the assembly including a low friction material for a pressure seal and a harder backup ring on the downstream side of the pressure seal to support it. The seal material is usually structurally weak and a strong back up ring material is used to prevent seal extrusion at the high pressures.
One source of problems with fluid swivels of the type described above, arises from extrusion of the dynamic side or edge of the soft seal material past the hard backup ring. The large pressures of thousands of psi on the seal can cause deformation or creep of the seal material past the backup ring, leading to the extrusion of the seal into the gap passage. Such extrusion generally results in seal failure and can only be prevented by keeping the gap between the backup ring and swivel metallic sealing faces very small. This requires the backup ring to be closely toleranced to the width of the swivel seal cavity. Also, when the swivel is pressurized, the deformations of the seal cavity must not cause the backup ring to be tight or loose in the cavity. If the backup ring is too tight, it causes high friction which leads to high swivel torque and/or backup ring failure. If the backup ring it too loose, the seal may not be properly constrained, causing it to creep and extrude with eventual failure.
Another source of problems is that high compressive loads can be produced between the backup ring and the dynamic swivel seal face as a result of the leaking of pressured fluid into the region between the pressure seal and the backup ring. The leaked fluid can result in a large pressure drop across the width of backup ring. The pressure drop presses the backup ring toward the swivel face that is sealed against, causing high friction at the dynamic side of the backup ring. U.S. Pat. No. 4,647,076 by Pollack (the present inventor) and Mann describe this very important problem encountered in operation of large fluid swivels.
One partial solution for minimizing extrusion of the seal past the backup ring, is to construct the adjacent surfaces of the swivel parts to close tolerances, so the width of the extrusion gap is very small. Of course, the need to manufacture to close tolerances results in high cost, and even such close tolerances do not avoid extrusion in many circumstances. A fluid swivel with a seal arrangement constructed to minimize extrusion of the backup ring, which was effective in avoiding such extrusion in a large number of circumstances without requiring costly manufacture of the swivel parts to close tolerances, would be of considerable value.